Triterpenic acids including ursolic acid are known to have various physiological activities such as antioxidant effect, anti-inflammatory effect, melanin production inhibitory effect, and collagen reconstruction effect, and there are known techniques for using a triterpenic acid as an active ingredient of the skin external preparation such as cosmetics (for example, Patent Documents 1, 2, 3 and 4). However, the triterpenic acid has poor solubility in an oily component and an aqueous component, and has a problem of precipitation of the triterpenic acid when the preparation is stored for a long period of time. Lack of storage stability of the skin external preparation containing the triterpenic acid is not preferred, because the above-mentioned physiological activities are reduced. Under such circumstances, there has been studied a method of improving solubility of the triterpenic acid in the oily component through derivatization of the triterpenic acid into an ester or ether (Patent Document 5). The method provides an effect of improving the solubility of the triterpenic acid in some degree, but the effect is not enough to prevent the precipitation of the triterpenic acid when the preparation is stored for a long period of time. In order to solve such problems, a technique of using aliphatic saturated alcohols having 12 to 24 carbon atoms or the like in combination has been developed (Patent Document 6). The technique provides the effect of improving the solubility of the triterpenic acid in some degree, but in the case where a preparation containing a triterpenic acid at a concentration of as high as 0.1% by mass or more is stored for a long period of time under a low-temperature condition, there may cause crystals to appear. That is, techniques for further improving the solubility and storage stability of the skin external preparation, containing the triterpenic acid have been required.
Meanwhile, 4-n-butylresorcinol is known to have a strong inhibitory effect of tyrosinase activity and to act as a very effective skin-whitening agent (Patent Document 7), and is used as a material of cosmetics. However, it has not been known that a 4-alkylresorcinol such as 4-n-butylresorcinol has the effect to enhance long-term storage stability of the triterpenic acid. Also, it has not been known that a 4-alkylresorcinol can be actually blended with the triterpenic acid in the skin external preparation.    Patent Document 1: JP 8-165231 A    Patent Document 2: JP 8-208424 A    Patent Document 3: JP 11-12122 A    Patent Document 4: JP 2000-302659 A    Patent Document 5: JP 09-143050 A    Patent Document 6: JP 2004-331593 A    Patent Document 7: JP 02-49715 A